This invention relates to superconducting analog amplifiers, and more particularly to superconducting analog amplifiers employing Josephson junctions.
Superconducting electronic circuitry lacks active three-terminal devices equivalent to semiconductor transistors. Existing superconducting active devices are based on Josephson junctions (JJs) which are two-terminal devices, and hence have relatively low gain. A high-gain superconducting amplifier can be produced by connecting a sufficiently large number of Josephson junction stages in series (i.e., cascading them). The rapid progress of superconducting LSI integrating technology has made this approach quite practical. However, to date, there is no known general-purpose high-gain superconducting amplifier (or a combination thereof) designed to have properties equivalent to those of a semiconductor operational amplifier (OP-AMP).
Known Josephson junction analog signal amplifiers include the use of direct current (dc) connected superconducting quantum interference devices (dc SQUIDs). SQUIDs have been extensively studied and their properties are described in numerous publications. By way of example, reference is made to the following articles: (a) K. K. Likharev, "Dynamics of Josephson Junctions and Circuits", Chapter III, Gordon and Breach Science Publishers, 1986; and (b) John Clarke, "SQUIDS: Principles, Noise and Applications", In: "Superconducting Devices", edited by S. T. Ruggiero and D. A. Rudman, Academic Press, Inc., 1990; These teachings are incorporated herein by reference.
Referring to FIG. 1a, there is shown a 2-junction superconducting quantum interferometer device (2-junction dc SQUID). The 2-junction dc SQUID consists of a superconducting loop 10 containing two non-hysteretic Josephson junctions (J1, J2) and two inductors (L1,L2). The loop 10 includes two branches connected in parallel between nodes 13 and 15; one branch includes J1 and L1 connected in series between nodes 13 and 15 and the other branch includes J2 and L2. A bias current Ib is supplied to node 13 to bias J1 and J2 and node 15 is grounded. Loop 10 is magnetically coupled to a superconducting control line 11 which includes inductive elements L1' and L2' which are magnetically coupled to L1 and L2. In the description to follow and in subsequent drawings, the SQUID of FIG. 1a may be drawn as shown in FIG. 1b. The voltage (V) across nodes 13 and 15 of the SQUID is sensitive to change in the magnetic flux. The control line 11 modulates the amount of magnetic flux in loop 10 which, in turn, modulates the voltage V between nodes 13 and 15 of the SQUID.
As illustrated in FIG. 1b, the voltage V across the interferometer is a function of the bias current I.sub.b into the node 13 and the magnetic flux .o slashed..sub.e created by the current I.sub.e in the control line 11.
FIG. 1c shows schematically a typical current-voltage (I-V) curve for different values of control flux .o slashed..sub.e in the interferometer. FIG. 1d shows typical voltage (V/Vc) versus flux .o slashed..sub.e /.o slashed..sub.0 curves for different values of bias current I.sub.b for the interferometer. The curves in FIG. 1d show that the SQUID response to the control magnetic flux .o slashed..sub.e is periodic. The period is a fundamental physical constant known as magnetic flux quantum .o slashed..sub.0 =h/2e=2.07.times.10.sup.-15 Wb. The maximum output voltage amplitude for a typical dc SQUID is of the order of 100 .mu.V.
FIG. 2a shows a prior art single-stage SQUID analog amplifier with a voltage output. The circuit is like that of FIGS. 1a and 1b with the addition of a load resistor RL between node 13 and a point of reference potential 15. FIG. 2a contains a single SQUID biased by a dc current (I.sub.b). The input signal is applied in the form of magnetic flux, typically via high-turn-ratio superconducting transformer.
FIG. 2b shows a more complex prior art analog SQUID amplifier which employs a series array of N identical SQUIDs, so the maximum output voltage (Vo) between nodes 130 and 15 is N times larger than that of a single-SQUID amplifier. Note that the power gain of the FIG. 2a circuit is the same as that of the circuit in FIG. 2b, because the SQUIDs in FIG. 2b share the input signal instead of cascading it. However, the circuit of FIG. 2b does have an advantage in that it provides the possibility of matching the output impedance of this amplifier (at node 130) with semiconductor room-temperature devices, which is very difficult to achieve with a single SQUID.
FIG. 2c shows a prior art cascaded 2-stage SQUID amplifier. The first stage of FIG. 2c is a voltage-biased SQUID with the flux output working as an active flux transformer. The second stage uses an array of N SQUIDs connected in series between voltage output node 132 and ground. The advantages of cascaded amplifiers are their higher gain (the gains of the stages are multiplied) and the possibility to optimize different stages for different functions; e.g. the first stage can be optimized for lower noise, while the second stage--for matched impedance.
The small-signal power gain of SQUID amplifiers (per stage) depends on the signal bandwidth; the maximum gain is inversely proportional to the bandwidth, and reaches unity in the GHz band. Wide-band high gain SQUID amplifiers can be obtained by cascading large number of stages. The main disadvantage of this approach is the possible mutual phase locking of the SQUIDs in different stages, which is difficult to suppress. This phenomenon is caused by the ac Josephson effect present in all Josephson devices and leads to increased noise.
Another prior art active device suitable for implementation of superconducting amplifiers is the exponential Josephson transmission line (JTL), also known as the "exponential flux shuttle". Such a device and applications thereof are shown or suggested in a) O. A. Mikhanov, V. K. Semenov and K. K. Likharev, "Ultimate Performance of RSFQ Logic Circuits", IEEE Trans. Magn., MAG-23, 759-762, 1987; and b) M. Gershenson, "Current Amplifier and Flux-Buffer Designs Using an Exponential Flux Shuttle with a Josephson Junction Synthetic Inductor", IEEE Trans. Magn., MAG-25, 1158-1161, 1989. The exponential JTL as shown in FIG. 3A is composed of N single-junction cells, each cell (Ci) detailed in FIG. 3B, consisting of an overdamped Josephson junction (Ji), with a critical current (I.sub.c), a superconducting inductor (Li) and a source of bias current (I.sub.bi). The critical currents of the junctions in the JTL increase exponentially along the line (I.sub.c.sup.i+1 /I.sub.c.sup.i =q, q&gt;1), while the ratios e=2.pi.I.sub.c L/.o slashed..sub.0 and b=I.sub.b /I.sub.c remain constant. The voltage gain of the JTL is unity (its input and output are connected via superconducting path), however its current gain can be very large. Current amplification is achieved when I.sub.inp exceeds the input critical current I.sub.ci of the JTL, while I.sub.out remains below its output critical current I.sub.co, so the fluxons are created at the JTL input stage and then flow through the JTL to the output.
An advantage of the JTL-based amplifiers is the easy cascading of stages without "phase-locking" complications of the SQUID-based amplifiers mentioned above; this makes it possible to use them to achieve arbitrary large power gain. The disadvantage of these amplifiers is their very low output impedance, which makes it difficult to match the amplifier output with most loads.
Two JTLs of the type shown in FIG. 3A may be interconnected as shown in FIG. 4. to produce what has been referred to as a "flux buffer" device for analog amplification. This circuit consists of two JTLs (JTL1 and JTL2) with inputs (IN1 and IN2) and outputs (OUT1 and OUT2) mutually connected via superconducting inductors (L.sub.INP and L.sub.OUT). The device functions as an active flux transformer; the Josephson junctions of the JTLs are typically biased slightly above their input critical current, so there is a flux flow along both JTLS. In response to changes of the flux in the input inductor the mutual phase of the two fluxon trains in the two JTLs is continuously changed, so the average flux residing in the output inductor follows the input flux. The energy and power gain of this device at low signal frequencies is close to the ratio of the input inductor to the output one and can be quite large.
The circuit of FIG. 4 has certain desirable features but it suffers from at least the following problems:
a) No means are provided for obtaining a voltage output. Only magnetic flux output is made available; PA1 b) The power gain of the circuit is limited to the ratio of the input inductor to the output inductor; and PA1 c) The circuit has considerable amount of ac oscillations at its output and provides no means for suppressing them. This would prevent cascading of such devices due to phase locking.